1. Field of the Invention
This invention relates generally to devices for removing floating oil from water.
More particularly, this invention relates to improvements in the construction and operation of oil skimmers, especially oil skimmers of the type using a rotating drum to pick up oil from the surface of a body of water.
In a specific embodiment, this invention relates to the use of certain materials and compositions as the oil contacting surface to enhance the capacity and efficiency of the skimming device.
In yet another embodiment, this invention relates to an improved process for operating oil skimmers allowing a substantial increase in their oil recovery capacity.
2. Description of the Related Art
The cleanup of crude oil and refined petroleum products spills on inland waterways, harbors and at sea is of ever increasing environmental and regulatory concern. Traditionally, three approaches have been taken to dealing with such hydrocarbon spills. The first approach has been to let nature take its course with the hydrocarbon either evaporating or dispersing into the water column. A second approach has been to apply chemical dispersants to the spill causing it to disappear from the water surface and spread within the water column. The last approach has been to collect the spilled hydrocarbon and remove it from the water surface using mechanical means; typically skimmers of one sort or another.
Doing nothing to clean up an oil spill is becoming increasingly unacceptable. The environmental impact of chemically dispersing oil in the water column is coming under more question. Consequently, the mechanical collection and removal of spilled hydrocarbons is now a favored approach to dealing with oil spills. The most commonly used systems for the mechanical recovery of spilled oil use skimmers which may be of the rotating disc, rotating drum, or belt type. All of those skimmers have one element in common. That is, a member having a surface arranged to be preferentially wet by oil rather than water and moving alternately into and out of the oil layer. As the member contacts oil, a film or layer sticks or clings to the surface of the member and is then carried by movement of the member out of the oil layer to a recovery zone where the oil is removed. Removal of oil from the surface of the member is typically accomplished by mechanical means such as, for example, a scraper or roller contacting or compressing the oil wet surface.
There are a number of variations of rotating disc skimmers disclosed in the prior art. Illustrative patents include U.S. Pat. No. 4,021,344 which discloses a skimmer having a plurality of discs fabricated of a plastic such as rigid polyvinyl chloride with the disc axes arranged about the circumference of a circle and driven from a central drive member. The discs pass through the oil-water surface and pick up oil which is removed from the discs by a scraper and pumped to a collection point. Another skimmer of different configuration but of generally similar mode of operation is described in the Morris U.S. Pat. No. 4,172,036. Morris also prefers to use discs of polyvinyl chloride and teaches that the oil adhering quality of the discs is improved by sanding the disc sides with 600 grit sand paper.
Likewise, there are a variety of rotating drum skimmers described in the patent literature. Examples here include patents to Kilpert et al, U.S. Pat. No. 3,426,902; to Delons et al, U.S. Pat. No. 4,681,680; and to Wilson et al, U.S. Pat. No. 4,957,636. All three patents teach the use of a drum surface having hydrophobic characteristics. Kilpert et al use a deformable spongy-type material for the drum surface preferring sponge plastics such as polyethylene, polypropylene or polyurethane for that purpose. The sponge cover is compressed by a roller just before it contacts the oil layer and draws oil into the sponge as the member expands during its passage through the oil. The oil filled sponge is then compressed by a second roller to wring oil from it for collection. Delons et al teach the use of a drum constructed of a hydrophobic-oleophilic material and patentees suggest for this purpose stainless steel, black steel, aluminum or a plastic material. Wilson et al disclose the use of such oil wettable and corrosion resistant materials as polyethylene and polypropylene for drum construction.
Mechanical skimmers tend to pick up some water along with the recovered oil. The amount of water picked up increases as the speed which the contacting member travels through the oil and water increases. That behavior is generally true whether the contacting member is a disc, a drum, a belt or of some other configuration. It is desirable that spilled oil and other hydrocarbons be recovered as free of water contamination as possible as reclamation and reuse of the recovered oil is facilitated and the storage and transport facilities at a spill scene are ordinarily limited. Further, there is a limit to the rate of fluid pickup by the contacting member without the fluid being flung off by the movement of that member. Hence, skimmers are ordinarily operated at rates below that of maximum fluid pickup in order to limit the amount of water recovered with the oil.
A major limitation with most existing skimmers is that the equipment is heavy and bulky in comparison to its skimming capacity making transport to the spill site difficult. Further, the performance of many skimmers is strongly dependent upon the sea state and upon the viscosity of the hydrocarbon being recovered. Consequently, the advantages of a light, easily deployed, large capacity skimmer capable of recovering oil essentially free from water can readily be appreciated.